How do I sanitze? Pose Library: Error: Potentially corrupt library, run 'Sanitze' operator to fix.
The Sanitize button (and command) did not fix the problem for me.

However, this did:


In Object mode I selected the armature. 
I switched to Pose mode, and entire armature was selected. 
I hit the spacebar to pull up the command browser, and typed in "pose" to filter the list of commands.
I had tried all the top ones, and it "felt" like I needed to do
something to the "PoseLib" so I chose "PoseLib Add Pose" 


The error is now cleared, and I have a Pose Library with a single "Pose" in it.

Blender 2.8 beta